The present disclosure relates to multilayer coating film structures and methods for forming a multilayer coating film.
In the coating of metal products requiring resistance to weathering, a multilayer coating film structure is typically used which includes an intermediate coating film formed on an anticorrosive electrodeposition coating film (undercoating film), and a top coating film superimposed on the intermediate coating film. For example, in the coating of an automotive body, an undercoating (electrodeposition coating), an intermediate coating, and a top coating (a base coating and a clear coating) have been conventionally applied to the automotive body in a sequential order, and a solvent-based paint has been used for the intermediate coating and the base coating. The intermediate coating film is provided to increase the light resistance, etc. For example, when an undercoating film made of an epoxide-based cationic electrodeposition paint is exposed to high doses of ultraviolet radiation, a surface part of the undercoating film is deteriorated, thereby separating a coating film on the undercoating film from the undercoating film. To address this problem, the electrodeposition film is protected from ultraviolet rays by the intermediate coating film.
By contrast, in some cases, in view of savings in resources, simplification of the process steps, cost reduction, etc., such an intermediate coating film is omitted, and a top coating film is superimposed directly on an electrodeposition coating film. Specifically, a water-based paint is used for a base coating, the water-based coating includes two layers of a first base coating film and a second base coating film, and light interception characteristics are imparted to the first base coating film, thereby omitting an intermediate coating.
Incidentally, as a method for coating an automotive body, the following method has been used: a base paint is applied to an inner plate portion, such as a pillar on the inside of doors and an engine compartment wall inside a hood; then outer plate members, such as the doors and the hood, are fitted onto the inner plate portion; and in this situation, a base paint is applied to entire outer plate portions of an automotive body including the outer plate members. Japanese Patent Publication No. 2003-93966 describes a measure for solving one problem of the above-described coating method. The problem is that when a base paint is applied to the vicinity of the border between the inner plate portion and each of the outer plate portions twice, this increases the film thickness, and that adhesion of paint dust flying from a coating gun to the inner plate portion during the coating of the outer plate portions causes viewers to feel something different about the hue of a shaded portion of the inner plate portion. To solve the above problem, the shaded portion of the coating film for the inner plate portion and highlight portions of the coating films for the outer plate portions provide substantially the same hue.